Blood coagulation is a process consisting of a complex interaction of various blood components, or factors, which eventually gives rise to a fibrin clot. Generally, the blood components that participate in what has been referred to as the coagulation "cascade" are proenzymes or zymogens, enzymatically inactive proteins that are converted to proteolytic enzymes by the action of an activator, itself an activated clotting factor. Coagulation factors that have undergone such a conversion are generally referred to as "active factors," and are designated by the addition of a lower case postscript "a" (e.g., factor VIIa).
Two systems promote blood clotting and thereby participate in normal hemostasis. These systems have been referred to as the "intrinsic" and the "extrinsic" coagulation pathways. It is now believed that the intrinsic pathway plays a role in the growth and maintenance of fibrin formation and that the "extrinsic" pathway is an overlapping mechanism that is critical for the initiation of fibrin formation. The pathways converge at the activation of factor X to Xa and proceed through a "common" pathway to fibrin formation. After vascular injury, tissue factor initiates the "extrinsic" coagulation pathway by complexing with factor VII in a calcium-dependent manner to facilitate the conversion of factor VII to VIIa. The factor VIIa-tissue factor complex can directly activate factor X to Xa. The intrinsic pathway may be activated by the generation of thrombin or factor XIIa which cleaves factor XI to generate factor XIa, the required enzyme for the initiation of the "intrinsic" coagulation cascade.
Fibrin formation via the "extrinsic" pathway is controlled by the presence of tissue factor pathway inhibitor protein (TFPI) which regulates the pathway in a factor Xa-dependent manner. TFPI, a multivalent Kunitz-type inhibitor, is believed to regulate the extrinsic pathway by forming a quaternary complex with factor Xa, tissue factor and factor VIIa, thus inhibiting the formation of free factor Xa and factor VIIa (Broze et al., Biochemistry 29: 7539-7546, 1990; which is incorporated by reference herein in its entirety).
In some instances, for example, kidney dialysis, deep vein thrombosis, and disseminated intravascular coagulation (DIC), it is necessary to block the coagulation cascade through the use of anticoagulants, such as heparin, coumarin, derivatives of coumarin, indandione derivatives, or other agents. A heparin treatment or an extracorporeal treatment with citrate ion (U.S. Pat. No. 4,500,309) may, for example, be used in dialysis to prevent coagulation in the course of treatment. Heparin is also used in preventing deep vein thrombosis in patients undergoing surgery. Treatment with low doses of heparin may, however, cause heavy bleeding. Furthermore, because heparin has a half-life of approximately 80 minutes, it is rapidly cleared from the blood. Because heparin acts as a cofactor for antithrombin III (AT III), and antithrombin III is rapidly depleted in DIC treatment, it is often difficult to maintain the proper heparin dosage, necessitating continuous monitoring of AT III and heparin levels. Heparin is also ineffective if AT III depletion is extreme. Further, prolonged use of heparin may increase platelet aggregation, reduce platelet count, and has been implicated in the development of osteoporosis. Indandione derivatives may also have toxic side effects.
In addition to the anticoagulants briefly described above, there are a variety of compositions disclosed within the art that are alleged to have anticoagulant activity. One such composition is disclosed by Reutelingsperger et al. (Eur. J. Biochem. 151: 625-629, 1985) who isolated a 32,000 dalton polypeptide from human umbilical cord arteries. Another composition is disclosed by Warn-Cramer et al. (Circulation Suppl, part 2, 74: 2-408ii, Abstract #1630, 1986). They detected a factor VIIa inhibitor of an apparent molecular weight of 34,500 in plasma.
Protein inhibitors are classified into a series of families based on extensive sequence homologies among the family members and the conservation of intrachain disulfide bridges (for review, see Laskowski and Kato, Ann. Rev. Biochem. 49: 593-626, 1980). Serine protease inhibitors of the Kunitz family are characterized by their homology with aprotinin (bovine pancreatic trypsin inhibitor). Aprotinin is known to inhibit various serine proteases including trypsin, chymotrypsin, plasmin and kallikrein. Kunitz-type inhibitor domains have been reported in larger proteins such as the inter-.alpha.-trypsin inhibitors (Hochstrasser et al., Hoppe-Seylers Z. Physiol. Chem. 357: 1659-1661, 1969 and Tschesche et al., Eur. J. Biochem. 16: 187-198, 1970), the .beta.-amyloid protein precursor and the .alpha..sub.3 -collagen type VI (Chu et al., EMBO J. 9: 385-393, 1990). TFPI (also known as extrinsic pathway inhibitor (EPI) or lipoprotein-associated coagulation inhibitor (LACI)) is a plasma protease inhibitor that consists of three tandem Kunitz-type inhibitors flanked by a negatively charged amino terminus and a positively charged carboxyl terminus. The first and second Kunitz-type domains have been shown to inhibit factor VIIa and factor Xa activity, respectively.
There is still a need in the art for improved compositions having anticoagulant activity that do not produce the undesirable side effects associated with traditional anticoagulant compositions. The present invention fulfills this need, and further provides other related advantages.
It is therefore an object of the present invention to provide novel human protease inhibitors of the Kunitz family of inhibitors with similar inhibitor profiles for use as anticoagulants and in the treatment of deep vein thrombosis and DIC.